Process for the complete bromination of non-fused ring aromatic compounds

ABSTRACT

There is disclosed a process for the preparation of completely brominated derivatives of aromatic compounds comprising one or more phenyl groups which may contain substituents, side chains or be partially-brominated. The complete bromination is effected by utilizing liquid bromine as a reaction solvent whereby the starting aromatic material is reacted with an excess of bromine which contains a bromination catalyst and the mixture is refluxed for a sufficient time. Use of these brominated derivatives as fire retarding agents is nylon and polyester is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

.[.This application is a division, of patent application Ser. No.80,122, filed Oct. 12, 1970, now abandoned..].

.Iadd.This is an application for reissue of U.S. Pat. No. 3,965,197,issued June 22, 1976, on application Ser. No. 222,412, filed Jan. 17,1972, which is a division of application Ser. No. 80,122, filed Oct. 12,1970, abandoned. .Iaddend.

The present invention relates to completely brominated derivatives ofaromatic compounds and particularly to the process for the preparationof said derivatives, said process being characterized by utilizingliquid elementary bromine as a reaction solvent. Moreover, the inventionis concerned with the utility of these brominated derivatives asfire-retarding agents in synthetic polymeric systems such as polyesterand polyamide.

Aromatic hydrocarbons comprising one or more benzene rings have beenpartially brominated by various methods. In fact, introducing up tothree bromine atoms per one aromatic molecule is usually accomplishedwith relative ease to the extent that a number of brominating agents canbe used. Difficulties are encountered, however, when furtherbromination, beyond the tri-bromo level, is desired. Encountering suchdifficulties is not unexpected since it is known that substituted ringpositions tend to reduce the activity level of adjacent, butunsubstituted, positions. Thus more drastic and stringent reactionconditions are necessary to effect further introduction of bromine intothe ring. For example, in the prior art, to prepare hexabromobenzenefrom benzene, mono-, or dibromobenzene the bromination is conducted inconcentrated sulfuric acid containing 29% free sulfur trioxide. Thereaction time is lengthy, usually between 9 to 16 hours and is generallycarried at elevated temperatures, i.e. about 150° C. Reaction conditionsof comparable harshness are used to prepare other highly brominatedaromatic compounds such as tetrabromoaniline, octabromobiphenyl anddecabromobiphenyl, see U.S. Pat. No. 3,232,959.

The bromination of aromatic compounds utilizing bromine or otherbrominating agents is conducted generally in the presence of solventswhich provide convenient reaction media insofar as solubilities, ease ofhandling and reaction with some formed by-products as means of theelimination thereof. A number of suitable solvents has been proposedincluding halogenated aliphatic compounds, organic and inorganic acids.For example, carbon tetrachloride, chloroform, alkylene dihalidescontaining 2 to 4 carbon atoms such as ethylene-, propylene-, butylene-,isobutylene dichlorides or dibromides, acetic acid and concentratedsulfuric acid have been utilized. Oleum containing some free sulfurtrioxide proved to be a very good solvent. Evidently the aromaticcompound reacts first with the sulfuric acid to form the correspondingmonosulfonic acid and water. When bromine is introduced into thesolvent, it will react with the monosulfonic acid to form thebromo-derivative and hydrobromic acid. But two undesirable by-productsare constantly produced, i.e. water and HBr. Because the solvent mediumcontains free SO₃ which is a strong oxidizer the HBr is reoxidized tobromine with the additional formation of water. At this rate it can beseen that the concentration of the sulfuric acid is continuouslydecreased which in turn will affect adversely the sulfonation reactionwhich requires a strong acidic medium. It is for this reason thatsufficient amounts of sulfur trioxide are added to maintain the sulfuricacid concentration at the proper level.

In the oleum process, the reaction temperatures employed will ofteninfluence the sulfonation of aromatic compounds which already containsubstituents. This influence is observed in the ring position of theintroduced sulfo group. In this respect formation of m-sulfonic acids isundesirable and it is necessary, therefore, to control the reactiontemperature in such a manner as to eliminate the formation of theseby-products which are very difficult to brominate completely.

Use of solvents, other than oleum, in the preparation of brominatedderivatives of aromatic compounds has been somewhat limited because ofthe bromination of the solvent itself. Unless the brominated solvent isin itself desirable the process becomes expensive. In the event thesolvent cannot be brominated such as carbon tetrachloride or otherperchloro compounds, problems relating to decomposition, solubility,recovery of solvent and separations of finished products areencountered.

In order to produce completely brominated derivatives of aromaticcompounds in high yields processes different from the above werenecessarily indicated.

According to the present invention a process has been devised wherebycompletely brominated derivatives of aromatic compounds are preparedfrom the action of bromine solvent on the corresponding aromaticcompound, in the presence of a bromine-transfer catalyst. Thus bromineis used herein as a reactant-solvent. At this juncture mention should bemade that the present invention is not concerned with aromatic compoundshaving fused or condensed rings such as naphthalene, anthracene andsimilarly related compounds. The invention, however, is concerned witharomatic compounds having one or more benzene ring, which may havesubstituents, such as benzene, toluene, phenol, xylene, biphenyl,biphenylether, biphenyl sulfide, and the like. Also it is to beunderstood that the starting material may be partially brominated suchas brominated benzenes and biphenyls.

The particular advantages of the process of the present invention arethe absence of a solvent other than bromine, the relatively low reactiontemperatures, the high yields and the purity of the brominated products.The bromination is carried out in the presence of a bromination catalystsuch as iron, aluminum, their halides, iodine and mixtures thereof. Thequantity of the catalyst depends on the reactants used. For example ithas been found that about 2.5 g of AlBr₃ per 1 mole of benzene wasneeded to effect the complete bromination to hexabromobenzene. Belowabout 2.5 g/mol, only mono- and dibromobenzene could be obtained. Itappears that the amount of catalyst required depends greatly on theamount of water present, because of the hydrolysis of AlBr₃. Water,therefore, should be kept at minimum levels.

As to the hydrogen bromide produced during the reaction, it is easilyremoved in its gaseous state. As such the HBr may entrain some of thereactants, usually bromine which can be trapped and recycled to thereaction vessel.

Inasmuch as the present invention contemplates a process of generalapplication, it should be borne in mind that minor modifications may benecessary depending on the starting material, equipment availability anddesired product quality. The bromination reaction may generate a greatdeal of heat because of the exothermic nature of the reaction. Thus thetime needed to complete the bromination of the aromatic compound maydepend on the size of reaction vessel, the efficiency of the coolingsystem along with the stirring and mixing operation. Moreover the amountof catalyst used will also influence the reaction time.

The fact that bromine operates as a solvent as well as a reactant theamount thereof is expected to be in a good excess over stoichiometricrequirements. It has been found that an excess of 100% of bromine issatisfactory in that little or no under-bromination results. Of course,if desired, greater excess can be used. After the reaction is completethe excess bromine can be easily separated from the finished product. Itis believed that most, if not all completely brominated derivatives ofaromatic compounds are insoluble in liquid bromine. The final mixture,therefore, can be filtered under slight pressure. Alternatively thebromine can be steam-stripped out of the reaction vessel with littleloss. Air can also be used for that purpose. With steam, however,recovery of bromine was over 94%.

It is well known that to maintain a reasonable rate of reaction requiresa higher temperature as each additional bromine is substituted. This istrue in the oleum process which starts at ambient or lower temperaturesand as the bromination proceeds heat is supplied until the reactiontemperature reaches usually about 150° C. The process of the presentinvention starts similarly at ambient or lower temperatures at whichstage the bromination is rapid and exothermic. As the reaction subsides,indicating that the hydrocarbon has at least been partially brominated,heat is supplied to the reaction vessel until the bromine starts toreflux. Refluxing is continued until slightly after the reaction iscompleted. Completion of the reaction is observed when the evolution ofhydrogen bromide ceases. The additional refluxing is precautionary sothat no underbromination occurs.

In order to illustrate some of the advantages of the instant inventionthe following examples are provided and in which the proportions aregiven in parts by weight unless specified otherwise.

EXAMPLE I Preparation of Hexabromobenzene

To a 300 ml three-necked flask equipped with a reflux condenser,stirrer, a dropping funnel and a thermometer 425 g of liquid bromine wasadded. 0.5 g of AlBr₃ was also added to the mixture which was agitatedto allow the catalyst to dissolve in the bromine completely. Thebromine - AlBr₃ mixture was then cooled to about 15° C. and maintainedat that temperature during the gradual addition of 15.6 g of benzene.The bromination reaction is quite rapid and care should be taken so thatnot much bromine and benzene are entrained with the evolving hydrogenbromide. Thus the size of the reaction vessel and the efficiency of thecooling and condenser system are important in this respect.

After the benzene was added, about one hour, the need for cooling thereaction vessel was no longer necessary. In fact the mixture was allowedto warm up slowly and afterwards heat was supplied to bring the bromineto reflux to about 60° C. The reflux was maintained for an hour. Herethe amount of catalyst added and the stirring efficiency affect thelength of the reflux period; more catalyst and efficient stirringreduces the time of reflux considerably. The excess bromine was removedby blowing superheated steam (about 140° C.) through the reaction vesseland collecting the bromine in an outside vessel. Within a few minutesonly a thick slurry of hexabromobenzene remained which was filtered,washed with hot dilute hydrochloric acid, filtered and finally washedwith hot water and dried at 75° C. The yield of the light tancrystalline product was 96.7% based on benzene. Determination of bromineand IR spectrum showed the product to be essentially hexabromobenzene,said product having a melting point of 329°-330° C.

EXAMPLE II Preparation of Pentabromotoluene

To a similar apparatus as described in Example I 471 g of liquid bromine(151 mls) was added to the 300 ml flask. 2.0 g of AlBr₃ was also addedand allowed to dissolve in the bromine. 0.2 moles of toluene (21.35 mls)was gradually added to the cooled flask (about 10°-15° C.) making surethat sublimation or rapid evolution of HBr was not such as to entraintoo much of the reactants. The addition of the toluene was carried outover one hour after which the flask was allowed to warm up and later itwas heated until the bromine commenced to reflux (58° C.) at whichtemperature the reaction was continued until no further evolution of HBrwas observed (occasionally the temperature was slightly increased tomaintain continuous reflux).

The solid product was filtered after removing the bulk of bromine. Afterwashing with hot dlute HCl and hot water the product was dried at 105°C. Weight obtained was 93.5 g of slightly tan crystals which onanalytical examination showed to be pentabromotoluene m.p. 286°-287° C.

EXAMPLE III Preparation of Tetrabromo-p-xylene

To a similar apparatus as described in Example I, 220 g of Br₂ was addedand 1.0 g of AlBr₃ was dissolved therein. To the cold mixture 10.62 g ofp-xylene was added very slowly over about one hour. The mixture ofbromine, catalyst and xylene was then refluxed at about 60° C. for about2-3 hours until no HBr was detected. The solid product was filtered,washed in the same manner as described under Examples I and II. Theslightly yellow crystals weighed 39.6 g and analytical tests showed themto be tetrabromo-p-xylene having a melting point of 247°-249° C. and ayield of about 94%.

EXAMPLE IV Preparation of Pentabromophenol

a. From phenol

In this preparation 475 g of bromine in which 2 g of AlBr₃ wasdissolved. 18.8 g of phenol was slowly added and later the mixture washeated to about 60° C. at which temperature the bromine reflux wasmaintained for a period of 2 hours. HBr was observed to evolve veryrapidly in the early part of the reaction.

After removal of excess bromine the crystalline product was filtered andwashed as usual with hot dilute HCl and water respectively. The driedproduct had a melting point of 225°-227° C. and based on analytical datait was shown that the brominated derivative is pentabromophenol.

b. From anisole

To a similar apparatus as described in Example I 471 g of bromine and 2g of AlBr₃ were added and allowed to dissolve. 21.6 g of anisole wasadded slowly to the bromine-catalyst mixture. The reaction was carriedout in the same manner as described above. However instead of obtainingbrominated anisole, analytical data showed the product to bepentabromphenol. It appears that the bromination in this reactioncleaved the molecule at the oxygen bridge forming the phenolic product.

EXAMPLE V Preparation of Decabromobiphenyl

To a 500 ml three-neck flask equipped similarly to those used in priorExamples, was added about 80 ml of bromine (250 g). The flask was cooledto about 10° C. 30.84 g of biphenyl was added in very small amounts tothe bromine. To another portion of bromine (about 150 ml; total of twoportions was 725 g) was added 10 g of AlBr₃ which was allowed todissolve therein. After all the biphenyl was added to the first portionthen the second portion was added. It should be understood that theaddition of the bromine was effected via the dropping funnel, since thebiphenyl is solid. In all the experiments the apparatus is equipped witha stirrer and/or suitable means. The mixture was then allowed to warm upand later heated to 60° C. at which temperature the refluxing of brominewas observed. Bromine reflux was continued for about 4 hours and thesolid tan colored crystals were collected, washed with hot dilute HCland water, respectively. Analysis showed that the product contained84.77% bromine and had a melting point of 385°-389° C. with a yield of95%.

EXAMPLE VI Preparation of Decabromobiphenyl ether

To an apparatus similar to the one used in Example V 34.05 g ofbiphenylether and a total of 782 g of bromine (added in two portions,the second containing 5 g of AlBr₃) were reacted in the same mannerdescribed in Example V above. The reaction is quite rapid and slowaddition was necessary. The mixture was allowed to warm after theevolution of HBr slowed down. Heat was applied to the mixture until thebromine started to reflux. The solid-liquid bromine mixture was treatedas described in the prior Examples to separate the brominatedderivative. The final product obtained was yellowish in color. Analysisshowed the product to be decabromobiphenylether containing bromine tothe extent of 83.42% and oxygen=1.79%. The yield was also over 90% oftheoretical based on biphenylether.

EXAMPLE VII Preparation of Decabromobiphenylsulfide

In an identical apparatus as used in Example VI 37.6 g ofbiphenylsulfide was placed in the 300 ml three-neck flask. 780 g ofbromine in two portions were added; the second portion containing 5 g ofAlBr₃. Prior to the addition of the second portion the mixture was keptcool at about 10° C. Very strong evolution of HBr was observed. Thesecond portion was added after the reaction subsided. The entire mixturewas then refluxed for a period of about 4 hours. Additional 150 g ofbromine was added. The final brominated product was filtered off afterthe evolution of HBr ceased. Bromine was removed by steam as describedin Example I. The product was washed with hot dilute HCl and waterseveral times and was later dried at 60° C. The grayish color crystalsweighed 198.3 g and were analyzed to be decabromobiphenylsulfide;bromine content was 83.23%, S=2.65% O=negligible.

At this juncture mention should be made that other catalysts have beenused in experiments comparable to those described in Examples I-VII.These catalysts were aluminum metal, aluminum chloride, iron metal,ferric chloride and iodine. Of course some adjustments were necessarysuch as increasing the amount of liquid bromine and/or insuring that thefinal product was free of the chlorides or iodides. It is also importantto relate that the brominating process of the present invention is notdirected to aromatic compounds having fused or condensed rings such asnaphthalene, anthracene and the like.

The effect of halogens and/or halogen-containing compounds on retardingand extinguishing fires is well known. Many chlorinated and brominatedorganic compounds have been incorporated or added to many polymericsystems for fire retardation purposes. Selection of the particularhalogenated compound (s) for use with certain polymeric systems is noteasy but, indeed, the result of considerable research efforts. In thecase of fire retarding agents of the additive type it is significantthat the agent satisfies the requirements of being (a) compatible withthe polymeric system (b) not migratory, and (c) of comparable thermalstability. To be compatible specifies that the fire retarding agent canbe blended with the polymer without affecting its mechanical andchemical properties. Further, it should not be of such a nature as tocause the migration of its particles toward the surface of the polymer.Finally it is important that the fire retarding agent has adecomposition range similar to the polymer itself. For the lastmentioned property, it should be noted that should the fire retardingagent decompose or break-down prior to the decomposition of thepolymeric system then it will afford little, if any, fire retardation.In the same manner should the fire retarding agent commence to decomposeafter the break-down of the polymer, the time lag or delay may be suchthat the fire retarding agent will be of no value. Thus it is an idealproperty to provide a fire retarding agent possessing identicaldecomposition properties as the polymer. In practice one usually findsthat the decompositions of the two materials, i.e., the polymer and thefire retarding agent are closely related.

In view of the above it becomes apparent that the selection of asuitable fire retarding agent is not arbitrary, especially in the caseof high melting polymers or those requiring hot temperature processesfor their manufacture such as polyamide (nylon) or linear polyester,respectively.

Completely brominated derivatives of biphenyl compounds such asdecabromobiphenyl, decabromobiphenyl ether and decabromobiphenyl sulfidehave been found to possess excellent fire retarding characteristics.Tests of these compounds on polyamide and polyester have given excellentresults as will be shown hereinafter. The fire retardancy of thepolymers was determined by the Oxygen Index Method which is nowrecognized for use on plastic materials by the American Society forTesting and Materials (ASTM) of 1916 Race Street, Philadelphia, Penn.19103, and said test has been given the designation D 2863-70 (SeeAnnual Book of ASTM Standards -- effective May 8, 1970). The OxygenIndex, also known as the Limiting Oxygen Index (L.O.I.) is defined asthe minimum volume-fraction of oxygen in an atmosphere of oxygen andnitrogen, which is needed to sustain the candle-like burning of apolymeric specimen. For further explanation and theoretical discussion,reference is made to a paper by J. DiPietro and H. Stepniczka presentedat the Society of Plastic Engineers Conference, New York, N.Y., May 6,1970, and published in Plastic Engineer Society, Inc., Volume XVI, pp.463-468.

The Oxygen Index (O.I.) for air is 0.209. The higher the value the lesslikely it is for the polymeric system to burn or sustain burning. Infact it is known that polymer systems having O.I. values about 0.27-0.28can be considered self-extinguishing in nature.

In Table I below there are provided data on the fire retardancy of bothnylon and linear polyester, said data being expressed in terms of theOxygen Index. Synergists such as antimony trioxide can be added toenhance the fire retardancy.

                  TABLE I                                                         ______________________________________                                                    Fire Retarding                                                                Agent and      Sb.sub.2 O.sub.3                                                                      Oxygen                                     Polymer     Percent        %       Index                                      ______________________________________                                        Nylon                 0          0     0.202                                  Nylon       DBBPS.sup.(a),   6.1 0     0.214                                  Nylon       DBBPS.sup.(a),  12.2 0     0.229                                  Nylon       DBBPS.sup.(a),  18.3 0     0.239                                  Nylon       DBBPS.sup.(a),  24.4 0     0.250                                  Nylon       DBBPS.sup.(a),   6.1 1.2   0.221                                  Nylon       DBBPS.sup.(a),  12.2 2.4   0.251                                  Nylon       DBBPS.sup.(a),  18.3 3.6   0.280                                  Nylon       DBBPS.sup.(a),  24.4 4.8   0.303                                  Polyester (Dacron)*   0          0     0.212                                  Polyester (Dacron)*                                                                       DBBP.sup.(b),    5.0 0     0.259                                  Polyester (Dacron)*                                                                       DBBP.sup.(b),   10.0 0     0.293                                  Polyester (Dacron)*                                                                       DBBP.sup.(b),   20.0 0     0.372                                  Polyester (Dacron)*                                                                       DBBP.sup.(b),   10.0 2.0   0.385                                  ______________________________________                                         *Trade-Mark of E. I. DuPont De Nemours & Co., Inc.                            .sup.(a) Decarbromobiphenylsulfide                                            .sup.(b) Decarbromobiphenyl                                              

It is to be understood that the foregoing detailed description is merelygiven by way of illustration and that other variations may be madetherein without departing from the scope of the invention as defined bythe following claims.

What is claimed is:
 1. A process for the complete bromination ofaromatic compounds on the ring portion thereof, said aromatic compoundshaving one or more benzene rings and are selected from the groupconsisting of benzene, toluene, phenol.[.xylene.]., biphenyl,biphenylether and biphenyl sulfide, comprising the steps of:a. reactingthe aromatic compound with a large excess of liquid bromine in thepresence of a bromination catalyst selected from the group consisting ofiron, aluminum, .[.iodine,.]. iron halide, and aluminum halide, at fromabout 10° C. to ambient temperatures to form a reaction mixture with theevolution of gaseous hydrogen bromide, said bromine excess being atleast 100 percent of the stoichiometric amount of bromine required forthe complete bromination; b. thereafter refluxing said reaction mixture;c. maintaining said reflux condition for a time sufficient wherebyhydrogen bromide ceases to form; and d. separating the completelybrominated aromatic reaction product from the reaction mixture.
 2. Aprocess for the preparation of completely brominated derivatives ofaromatic compounds according to claim 1 wherein said brominationcatalyst is selected from aluminum halides, iron halides .[.andiodine.]..
 3. A process according to claim 1 wherein the reactionmixture formed in step a. is maintained about 10 degrees Centigrade tocontrol the initial bromination reaction.
 4. A process for thepreparation of completely brominated derivatives of aromatic compoundsaccording to claim 1 which is further characterized by having thereaction mixture stirred throughout the bromination reaction.
 5. Aprocess for the preparation of completely brominated derivatives ofaromatic compounds according to claim 1 wherein the derivative of step(d) is treated with super-heated steam to remove any occluded bromine.6. A process according to claim 1 wherein the completely brominatedaromatic reaction product is selected from the group consisting ofpentabromophenol, pentabromotoluene and tetrabromoxylene.
 7. A processfor the preparation of completely brominated derivatives of aromaticcompounds according to claim 2 wherein said catalyst is selected fromaluminum and iron chlorides.
 8. A process according to claim 1 whereinthe said completely brominated aromatic derivative is washed and dried.9. A process for completely brominating an aromatic compound selectedfrom biphenyl, biphenyl ether and biphenyl sulfide to produce thecorresponding decabrominated derivative, comprising the steps of:a.reacting the aromatic compound with liquid bromine in the presence of abromination catalyst selected from the group consisting of iron,aluminum, .[.iodine,.]. iron halide and aluminum halide at from about10° C. to ambient temperature to form a reaction mixture with theevolution of gaseous hydrogen bromide, said liquid bromine being presentin said mixture in an amount in excess of the stoichiometric amount ofbromine by an amount at least 100 percent of the stoichiometric amountrequired for the complete bromination; b. thereafter refluxing saidreaction mixture; c. maintaining said reflux condition for a sufficienttime whereby hydrogen bromide ceases to form; and d. separating theformed decabrominated derivative from the reaction mixture.
 10. Aprocess according to claim 9 wherein the reaction mixture formed in stepa. is maintained at about 10 degrees Centigrade to control the initialbromination.
 11. A process for completely brominating an aromaticcompound selected from biphenyl, biphenyl ether and biphenyl sulfideaccording to claim 9 wherein the decabrominated derivative is treatedwith super-heated steam to remove any occluded elemental bromine.
 12. Aprocess for completely brominating an aromatic compound selected frombiphenyl, biphenyl ether and biphenyl sulfide according to claim 9wherein the reaction mixture is kept stirred throughout the brominationreaction.
 13. A process according to claim 9 wherein said brominationcatalyst is selected from aluminum halides.[.,.]. .Iadd.and.Iaddend.iron halides .[.and iodine.]..
 14. A process according to claim9 wherein said decabrominated derivative is washed and dried.